Well the dealership replaced the cat, but not the O2 sensors, and the CEL went away. Of course they also replaced the gasket in the exhaust system, which is also a possible source of the problem.
Most times O2 sensors don't go bad too quickly, unless you're burning something unusual, like oil or sometimes certain cleaners. Also using certain sealants in the intake system can result in fumes which harm O2 sensors (Most RTV is now marked "sensor safe" and does not produce these fumes.) I've also brought back some O2 sensors by heating it with a nice clean flame, like a propane torch and burning off contamination, but usually it's more cost effective to just buy a new sensor, so long as you've fixed the problem that caused it to foul.
Anyway, the dealership said it was the cat which failed because of the purge valve issue, which also got me a new gas tank.
Now I need to get my backup camera working again. This car has been a parade of problems so far...
2012 Focus Titanium Hatchback
121,000 km, all stock
1990 Toyota Supra turbo
I think the cams are still stock...